In an image forming device which uses a two-component powder developer constituted by a toner and a carrier, it is required to supply the toner to the developer on the basis of a result of detection of weight-mixture ratio of the toner to the carrier (hereinafter, referred to as "toner density"), so as to keep constant density of an image by compensating the toner consumed by image formation.
Therefore, conventionally, in a known method for measuring the toner density, the toner density in the developer is measured on the basis of amount of reflected light from the developer which is constrained by magnetic field, and which is illuminated through a transparent detection window (hereinafter, this method is referred to as "optical detection method"). Further, it is proposed as a kind of the optical detection method to make the transparent detection window and the toner electrically repulsive with each other by applying window bias having a polarity identical with that of the charged toner to the transparent detection window in order to prevent the toner from adhering to the detection window.
However, the charging amount in the developer varies with humidity and deterioration degree of the developer. More specifically, the charging amount decreases when the developer becomes wet on account of rise of humidity. On the other hand, the charging amount increases when the developer becomes dry on account of drop of the humidity. Further, the charging amount decreases with the deterioration of the developer caused by agitation etc., while the charging amount increases when the developer is not affected by the agitation etc., and when the developer is fresh.
When the charging amount decreases, attraction force between the carrier and the toner becomes weak, so that they separate from each other. In this case, the carrier does not adhere to the detection window because it is constrained by the magnetic field. However, the toner separating from the carrier is suspended, and this suspended toner adheres to the detection window because the charging amount of the toner is small and electrical repulsive force between the toner and the window bias is weak. On the other hand, in the case that the charging amount of the carrier becomes large, the toner does not adhere to the detection window because the repulsive force between the toner and the window bias becomes strong, whereas the carrier is released from the constraint of the magnetic field and adheres to the detection window because the electrical attraction force between the carrier and the window bias becomes stronger than the constraint force of the magnetic field.
In the case that when the toner adheres to the detection window, it is judged that the density of the toner is higher than reference density to be compared because the amount of the reflected light from the developer increases. Therefore, there arises a problem that the density of the image drops because the toner is not replenished in spite of dropping of real toner density. On the other hand, in the case that the carrier adheres to the detection window, it is judged that the toner density is lower than the reference density because the amount of the reflected light from the developer decreases. Therefore, there arises a problem that the developer overflows from a development apparatus resulting in contamination of the interior of an image forming machine because the toner is replenished excessively in spite of required toner density being actually maintained.